Inteniatiorial Joiiriial of Food Scieiices arid Nutritiori (2001) 52, 389-400

. Viscosity of gruels for infants: a comparison of measurement procedures r)I & Clair8’ Mouquet and Sergi Trèche 106 Research Unit - Nutrition, Alimentation, Sociktks, Centre IRD, BP 5045, 34032 Montpellier cedex, France

Numerous studies have been carried out to investigate energy density and consistency of gruels for infants in developing countries. However, starch-rich gruels have a complex rheological behavior and their consistency is difficult to characterize. Many published gruel viscosity data are available, but the lack of standardized viscosity measurement procedures makes comparisons and inter- pretations difficult. The influences of viscometer type and viscosity measur‘ement‘”‘* conditions on gruels prepared with simple or multicomponent flours were assessed in this study. The results showed a drastic decrease in apparent viscosity when the shear rate increased. Other factors like shear time and gruel temperature also had a marked influence on apparent viscosity. For two types of gruel (maize or multicomponent flour) prepared at different concentrations, coirespondences between a short qualitative description of the. consistency and apparent viscosity values obtained with several viscometers in different measurement conditions are given. Finally, recommendations are put forward on techniques to obtain valid data on gruel consistency, adapted to each type of study (laboratory, field or large-scale surveys).

Introduction

The importance of increasing gruel energy reported a positive impact of viscosity reduction density to improve the energy intake of young on the amount of gruel consumed (Gopaldas & children in developing countries is now fully John, 1992; Rahman efal., 1994; Darling et al., recognized (Brown, 1997; WHO, 1998). How- 1995; den Besten et al., 1998; Bennett et d., ever, increasing the energy density of these 1999) whereas others could not find any generally starch-rich gruels simply by increas- significant differences between high energy ing the flour concentration leads to drastic density-high viscosity versus high energy den- changes in gruel consistency during cooking, sity-low viscosity diets (Marquis et al., 1993; i.e. especially substantial thickening. Stephenson et al., 1994). This controversial Church (1979) postulated that gruel con- issue has been reviewed by several authors. sistency had an influence on the energy intake Ashworth and Draper (1992) concluded that the of young children. Many studies have thus effectiveness of malting (process used to reduce compared diets of low or high energy densities the viscosity of high energy density gruels) to and low or high viscosities. Some of them improve the energy intakes of young children ..

Correspondence to: Claire Mouquet.

.. i-- ISSN 0963-7486 printed/ISSN 146513478 onlink 01/050389-12O 2001 Taylor & Francis Ltd i DOI: 10.1080/09637480120078276 -- - - __ - - -. - S, 390 C. Mouquet & Trèche

the Table 1. Correlations between descriptive terms used in sensory methods and viscosity ranges proposed in literature

Number Short descriptive Qualitative Viscosity Viscometer type Rej of terin or definitiori rarige aiid measuring classes expression (Pa.s) conditions

3 drinkable <1 Haake VT500, 1 measuring system spoonablle with the consistency of 1< <3 SVDin, shear rate yoghurt = 83 P' thick >3 gruel temperature = 45°C

6 free-flowing <1 liquid soup-like I<<3 easily spoonable 3< <6 Brookfield LVT 2 thick, batter-lie 6< 40 measuring very thick, non- IO< <40 conditions not spoonable reported dough-lie >40

4 liquid/semi-liquid free-flowing state which <3 could easily be poured from a tablespoon soft could easily be poured in 3< 20 cannot be poured

Trèche and Mbome Lapé (1999)'. Gopaldas et al. (1988). Svanberg (1987).

was not clearly demonstrated. More recently, be only characterized by a constant viscosity Trèche (1996) and Brown (1997) pointed out value but raises several values that can vary the apparent inconsistency of all published markedly depending on measurement condi- results. According to Trèche (1996), imprecise tions, i.e. viscometer type, measurement spin- viscosity determinations could be a key factor dle, shear rate, shear time and gruel tem- hindering comparisons between these different perature. Many gruel viscosity data are studies and definitive conclusions on the effects available in the literature, but comparisons of gruel viscosity and energy density on infants' and interpretations are often unfeasible energy intake. because the measurement conditions are not Gruel consistency is usually characterized always given in detail and they can change into two ways: between studies. The second way is based on sensory evalu- The first consists of measuring gruel vis- ation and involves describing the gruel con- cosity. However, gruels are generally pre- sistency with qualitative adjectives. The pared with starch-rich raw materials and descriptors used, however, are highly varia- consequently have a complex rheological ble, e.g. the most diluted gruels are ranked as behavior. A specified gruel therefore cannot liquid, thin or fluid, while thick concentrated Viscosity of gruel for irlfaiits 391

gruels are classified as thick, stiff, sticky or Table 2. Composition of simple flour and semi-solid, etc. Moussa et al., 1992; Marquis multicomponent gruels et al., 1993; Rahman et al., 1994). Other authors use more flamboyant expressions colllpositioll based on comparisons with other foods Type of gruel (g/lOOg dry rilatter) (yogurt, thin milkshake, soup, pour batter or ~ Simple flour gruel drop batter, dough, mashed potatoes, etc.), or Gruel A Maize 100% on the way the characterized food product has Gruel B Rice 100% to be consumed due to its consistency (drink- Gruel C Cassava 100% able from a bottle or from a cup, spoonable), Multicomponent flour gruel (Gopaldas et al., 1988; Gopaldas & John, Gruel D Millet 68%, soybean 17%, 1992; Marquis et al., 1993). The main groundnut 5%, sugar 9%, problem linked to these descriptive methods salt 1% is that the choice of the descriptors is subjective and closely linked to the authors’ culture and dietary habits. Consistency can thus be described by a wide variety of words millet (68%), soybean (17%), groundnut (5%), or expressions and different consistencies are sugar (9%) and salt (1%)purchased in Ouaga- sometimes described by the same words. dougou (Burkina Faso) was also used.

Several authors have proposed different con- Preparation of gruels sistency classes, with correlations between Gruels at different concentrations were prepared descriptors and viscosity values (Table l), but in the following way. Flour was mixed with these classifications differ in many ways: num- cold demineralized water into a slurry and ber of classes, qualitative description used, and cooked with continuous stirring over a hot plate corresponding viscosity range. For instance, the for 5 min once the mixture started to boil. Then viscosity expressed by the term ‘spoonable’ the gruel was allowed to cool to a desired ranges from 1 to 3 Pas according to Trèche and temperature before viscosity measurements. Mbome-Lapé (1999), but from 3 to 6 Pas for The dry matter content of the gruel was Gopaldas et al. (1988). These variations show determined by oven-drying at 105°C to constant that conditions for assessing gruel consistency weight. need to be accurately standardized, with respect Composition of the four types of gruel tested to both instrumental (measurement conditions) is given in Table 2. Attention has to be paid to and sensorial methods (descriptors). the fact that those very simple gruels have been The aim of this article is to highlight the chosen to illustrate the complexity of the influence of viscometer type and measurement rheological behavior of such starch-rich gruels, conditions on apparent viscosity values of and are not satisfying from a nutritional point of starch-rich gruels and to put forward recom- view. mendations for the standardization of con- sistency characterization methods. Measure- Viscosity measurements ments were performed on several types of We performed measurements with three differ- gruels to illustrate the apparent viscosity ent systems selected from amongst those used dependency on the measuring system. Results in the literature: a Haake VT550 to control shear led us also to propose a set of measurement rate, a Brookfield RV which is the most conditions corresponding to usual consumption commonly used, and a Rion VT04, which is conditions. very simple and can be used easily in the field. These three apparatuses are rotational visc- ometers but the ‘fundamental’ ones have to be Materials and methods distinguished from the empirical ones. Flour samples In fundamental viscometers, the measure- Simple cassava, rice and maize flours were ment cells (coaxial cylinders or cone-plate obtained from local commercial suppliers. A systems) are characterized by narrow clearance multicomponent infant flour prepared with between the rotating member and the fluid- 392 C. Mouquet & S. Trècke

I Coaxial cylinders Cone-plate system I Rotating spindle '%unrlameiital" viscometers Empirical viscometers "Brookfield" type Uniform shear rate at all fluid points shear rate not controlled

Figure 1. Measurement systems of rotational viscometers. containing cup, thus the shear rate is controlled (Launay, et al., 1986) enables a description of in all points of the product (Figure 1). the viscous behavior over the whole range of The fundamental viscometer we used was the shear rates studied. Hake VT550 with SV-DIN coaxial cylinders All equation parameters and determination driven by a PC computer with the Rheowin 2.67 coefficients were determined by ordinary least- software. The measuring procedure involves squares regression. We used the modeling five steps: 10 min at 83 s-', 4 min at 8 sul, 4 min function of the Rheowin 2.67 software based on at 800s-' and a ramp-up from 0.01 to 900s-' the Levenberg (1944) and Marquardt (1963) followed by a ramp-down from 900 to 0.01 s-'. algorithm. The last values of the first three steps were kept Empirical viscometers such as the well- as apparent viscosities at 8, 83 and 800 s-'. The known Brookfield type are based on torque last step (rampdown) was fitted with the very measurements when a spindle of various sizes commonly used Ostwald de Waele (or power rotates in a container. The clearance between law) equation. the rotating member and the container wall is usually large, leading to a variable shear rate in o-=K.$J' the fluid (Figure 1). In this case, the controlled parameter is the rotation speed of the spindle Ostwald de Waele equation (1) and not the true shear rate. For non-Newtonian fluids, the use of these viscometers allows where u is the shear stress (Pa), and the 9 measurements of 'Brookfield-type' viscosity, shear rate (CI). which is only useful for comparative purposes. This equation can also be written: We performed 'Brookfield-type' measurements = K ?"-' with two apparatuses: one Brookfield RV model pap with spindles 4 and 6 at different rotation speeds Equation' (2) and the Viscotester Rion VT04 with spindle 1 at the only rotation speed possible with this where papis the apparent viscosity (Pas). apparatus, i.e. 62.5 rpm. The two constants of these equations, the All viscosity measurements were done in consistency index K (Pas), and the exponent 11 thermostated conditions, at 45 k OSOC, except (adimensional), also called flow behavior index when the temperature influence was studied. Viscosity of gruel for iilfaiits 393

Gruel A Gruel B Gruel C (maize) (cassava) (rice)

Apparent viscosity (Pas) Apparent viscosity (Pas) Apparent viscosity (Pa.s) I I II , 40 3 O- / 20

10

O 9 3 5 7 9 11 133 5 7 11 133 5 7 9 11 13 Concentration (g DW100g) Concentration (gDW100g) Concentration (g DW100g)

Figure 2. Influence of the concentration on the apparent viscosity of gruels A, B and C (respectively maize, cassava or rice flour) measured with different viscometer types (shear time = 10 min; gruel temperature = 45°C). + Haake VT500, 83 s-' ORion VT04 spindle, 1, 62.5rpm O Brookfield RVT, spindle 6, 20rpm

Measurement repeatability was evaluated by thinning properties. The shear rate applied was performing five measures on a maize gruel 83 s-' for the Hake system, and could not be (7.82%) with each of the eight viscosity meas- determined, but the mean values were probably urement systems used corresponding to the much lower for the two others. three apparatuses and the different conditions Figure 3 illustrates the importance of shear sets (shear rate or rotation speed and spindle rate on apparent viscosity values, under con- number). In each case, the coefficient of trolled shear rate conditions. Indeed, irrespective variation was below 8%. of the type of gruel (simple or multicomponent flour), the apparent viscosity drastically decreased as the shear rate increased and this Results and discussion trend was accentuated as the gruel dry matter Influence of gruel concentration content increased. This shear-thinning phenom- Measurements were done on gruels prepared enon is usually explained as follows: at a certain with maize, rice or cassava flour at different concentration threshold, starch macromolecules concentrations (Figure 2). are close enough together to establish low- Unsurprisingly, the results show a marked energy bonds, thus boosting the apparent vis- influence of the gruel dry matter content: the cosity of the gruel. Rotation of the viscometer apparent viscosity values increased very rapidly spindle energizes the system, which halts these with the dry matter content. But dry matter interactions, upsets the orientation and separates content is an intrinsic character of gruels that is any chains that offer shear resistance, leading to difficult to control especially in field studies. a decrease in the apparent viscosity. These interactions begin occurring again and the Influence measurement system of viscosity apparent viscosity rises as this energy input Figure 2 also shows clearly that the apparent . decreases (i.e. as the shear rate decreases). viscosity values of gruels are closely dependent To more accurately describe the rheological on the viscosity measurement system used. This properties of such shear-thinning products, it is due to the complex rheological behavior of has often been proposed to fit the apparent these starch-rich gruels. viscosity,curves over a wide range of shear rates Shear rate with the Ostwald de Waele model (Evans & This non-Newtonian behavior is primarily due Haisman, 1979; Sopade & Filibus, 1995). to a marked shear rate dependency called shear- Values of the two coefficients K and ia of models 394 C. MoLiquet & S. Trèche

Gruel A Gruel D Apparent viscosity (Pas) Apparent viscosity (Pas)

3i

O 200 400 600 800 ,O 200 400 600 800 1000 1000 shear rate (i1) shear rate (i')

Figure 3. Influence of shear rate on apparent viscosity of gruels A (maize flour) and D (multicosmponent flour) prepared at difference concentrations (shear time = 10min; gruel temperature = 45OC).

obtained for two types of gruels (maize and the apparent viscosity measured at 83 s-'. From multicomponent flours) prepared at different a practical point of view, although the quality of concentrations are presented in Table 3. the fit was, good, the results of the analysis of The determination coefficients (all higher the model coefficients were similar to those than 0.99) indicated a close fit between the obtained with a simple analysis of apparent model and experimental results. As reported by viscosity at a defined shear rate (e.g. 83 s-'). In al. Launay et (1986), the flow behavior index n many cases, the latter method (easier to carry was inferior to 1, thus indicating shear-thinning out) could be sufficient. behavior and decreased when the gruel concen- tration increased. As expected, the consistency Shear time index K increased as the gruel concentration The shear time is another important factor increased, but in a much more marked way than modifying the apparent viscosity, which

Table 3. Ostwald de Wade parameters and determination coefficients obtained by fitting flow cumes (between 0.05 s-') and 850 of gruels A (maize flour) and D (multicomponent flour) at various concentrations

Ostwald de Waele Apparent parameters Gruel viscosity Deternuhation concentration at 83 s-' K Il coeflcient (dl006) Pas Pa.s R2

Maize flour 5.9 0.16 0.042 0.68 0.9985 6.8 0.21 0.578 0.68 0.9992 8.7 1.05 3.934 0.59 0.9982 9.1 1.37 5.686 0.57 0.9979 10.3 2.41 11.41 0.54 0.9973 Multicomponent flour 9.1 0.16 0.428 0.69 0.9987 10.7 0.33 0.997 0.66 0.9989 11.4 0.54 2.378 0.59 0.9995 12.5 0.72 2.643 0.62 0.9995 13.4 0.89 3.631 0.60 0.9996 15.7 1.59 7.920 0.57 0.9996 Viscosity of gruel for infants 395

Gruel A Gruel B Gruel C (maize) (cassava) (rice) Apparent viscosity (Pa.s) Apparent viscosity (l'as) Apparent viscosity (Pas) 9 8 Uo 9.5% '6 16.2%

4 3 2 2

1 1 O O O 2 6 O 2 6 O 2 6 4 8 10 12 14 16 4 8 10 12 14 16 4 8 10 12 14 16 shear time (min) shear time (min) shear time (min)

Figure 4. Influence of shear time on apparent viscosity of gruels A, B and C prepared respectively with rice, cassava or maize flours at different concentrations (shear rate = 83 s-'; gruel temperature = 45OC).

decreases as shear time rises. The recording of that can show thixotropic behavior when pre- apparent viscosity value of simple flour gruels pared at high concentration. We can see in during 15 min illustrates this property, called Figure 4 that, almost non-existent at concen- thixotropy (Figure 4). Thixotropy is usually trations below 7%, the thixotropic behaviour attributed to the progressive breakdown of appears and becomes more and more pro- aggregates of suspended particles under a nounced for gruels of each type of simple given shear stress. As the number of disrupted flour tested when the gruel concentration interparticle bonds increases, the viscosity increases. The practical consequence of this drops. A balanced particle aggregation state shear time dependency is that the shear time corresponds to each shear rate. In a standing has to be set at a predetermined value before state, these particle aggregates reform and the measurements. These results led us to take medium begins restsucturing. Gruels are actu- apparent viscosity readings after 10 min shear ally suspensions of swollen starch granules time since we found that the apparent viscosity

o Gruel C, 8.0% ooO O~oQooo

70 60 40 50 30 Temperature (OC)

Figure 5. Variations in apparent viscosity of different types of gruels during cooling between 70 and 30°C (shear rate = 83 s'). S. 396 C. Mouquet di Trèche values started to stabilize from this shear time tuses or in different measurement conditions. for most of the gruels. It is interesting to note that the same viscosity value measured at 83s-' of 0.16 Pas corre- Gruel temperature sponding to dry matter content of 5.9 and 9.1 A third factor that could substantially modify (%, wt) for maize and multicomponent gruels viscosity is the gruel temperature, i.e. viscosity respectively, corresponds to different apparent generally increases as the gruel cools. This is a viscosity values when measured in other condi- very common phenomenon that occurs with all tions, e.g. with a Brookfield RVT spindle 6 at types of gruel, irrespective of the concentration 50 rpm (0.6 and 0.9 Pas, respectively). This can (Figure 5). It is therefore important to perform be explained by a shear-thinning behavior more viscosity tests under thermostatically controlled marked for multicomponent gruels than for conditions. The viscosity measurement tem- maize gruels and shows the limits of using perature should be set close to the temperature at single apparent viscosity measurements to which the gruel is usually consumed by infants. describe gruel consistency. This consumption temperature generally ranges from 401 to 50"C, but varies slightly in different geographical contexts. In the literature, the most Conclusion common temperatures at which viscosity meas- All the results presented previously indicate that urements are performed are 40°C (Svanberg, measurement conditions for assessing gruel 1987; Araya et al., 1991; Wanink et al., 1994) or consistency have not been sufficiently standar- 45°C (John & Gopaldas, 1988; Hayes et al., dized. Because of this lack of standardization, 1995; Trbche & Mbome Lapé, 1999). many published results cannot be compared with those obtained in other studies, thus often Comparison of viscosity measuring systems leading to contradictory conclusions conceming used in previously publislied studies the impact of gruel consistency on energy intake A wide variety of measuring systems have been (Rahman et al., 1994; Stephenson et al., used to evaluate the gruel consistency by 1994). different authors in different countries (Table 4). Eventually, what gruel consistency character- We noted that in the 35 published studies ization method could be recommended? There dealing with gruel consistency mentioned in is no universal answer, but several different Table 4,78% used an empirical viscometer with possibilities could be put forward, depending on a rotating spindle (Brookfield RV model, 42%; the type of study that is to be carried out: Brookfield LV model, 25%; other Brookfield, 8% and Rion VT04, 3%), and the other 22% * to investigate the effects of a process on gruel used one viscometer with coaxial cylinders consistency at a laboratory scale, the best allowing shear rate control (Haake or Rheomat). approach would be to conduct viscosity The extremely variable measurement conditions analyses under standardized conditions, i.e. applied (shear rate or spindle number and fixed shear rate (or measuring spindle number rotation speed, and gruel temperature, the shear and spindle rotation speed), and controlled time is generally not reported) explain the shear time and gruel temperature. The best is marked differences between measured viscosity to use, when available, a viscometer that ranges. In some cases, the measurement condi- enables shear rate control. In our laboratory, tions and even the viscometer type are not we have been using a Haake VT500 visc- reported. ometer for several years under the following In Table 5, we tried to establish, for maize conditions: shear rate = 83s-' (SVDIN and multicomponent gruels at several concen- coaxial cylinders; 64.5 rpm); gruel tempera- trations, conespondences between apparent vis- ture = 45°C and shear time before readings = cosity values obtained with different measuring 10 min. These measurement conditions were systems and qualitative descriptions of the gruel chosen for the two main following reasons. consistency consensually determined by our First, they match conditions in which infants research team. The results clearly show that it is perceive gruel consistency: 45°C is close to vain to compare apparent viscosity of gruels the temperature at which gruels, are con- when they are measured with different appara- sumed, and 83 s-' is within the range of shear 4.

Table 4. Comparison of measurement procedures used for gruel viscosity characterization in the literature

Measurement conditions Approxinzate Viscometer type Measiiriiig Shear rate (s-') Gruel viscosi? system or or rotation speed teinperatitre range Trade nzark Model spindle (rP4 (OC) Cozintry Pa.s Bibliographic reference

Brookfield RV 2 20 rpm 28; 60 Tanzania 0.5-0.8 Darling et al. (1995) 2; T-SP. 2.5 rpm n.r. Bangladesh 2-700 Wahed et al. (1994) 2; T-sp. 2.5 rpm n.r. Bangladesh 1.7-503 Mahalanabis et al. (1993) 3 20 rpm ambient USA 0.03-31 Likimani et al. (1991) 4 20 rpm 40; 60 Peru 0.3-5 Marquis et al. (1993) 5 50 rpm 45-50 USA 0-8 Jansen et al. (1981) 6 20 rpm n.r. Tanzania 3-50 Mosha & Svanberg (1990) 6 50 rpm 40 Chili 9 Alvina et al. (1990) 6 50 rpm 40 Chili 3-9 Araya et al. (1991) 5; 7 50 rpm 45-50 Zambia 0.001-75 Hayes et al. (1995) 7 50 rpm 45 India 5-40 John & Gopaldas (1988) ar.- 10,20,50,10Orpm 30,40,50,60,70 Nigeria 0-1 10 Sopade & Filibus (1995) n.r. lOOrpm 30 India 0.25-15 Malleshi & Desikachar (1988) n.r. 60 rpm 30 India 0.9-1.1 Krishna Kumari & Geervani (1996) LV 3 30 rpm 45 India 7-37 Gopaldas et al. (1986) 1, 4 0.005-1 s-l 25 India 5-65 Changala Reddy et al. (1989) 4 6 rpm 40 Tanzania 1-70 Gimbi et al. (1997) various sp. 60 rpm ambient India 0-10 Malleshi & Desikachar (1982) n.r. n.r. 40 Nigeria 0.1-10 Akpapunam et al. (1996) n.r. n.r. n.r. India 0.005-0.015 Gopaldas et al. (1982) n.r. n.r. n.r. India 0.01-9.5 Chandra Sekhar et al. (1988) n.r. 12rpm 30 India 0.15-4 Kulkami et al. (1991) HB SC4-21 50 rpm 40 USA 0.7-5 Griffith et al. (1998) DVII B,E 3 rpm 50 Peru 2.5-570 Bennett et al. (1999) n.r. n.r. n.r. 35-40 India 1.7-60 Gopaldas et al. (1988) Rion VT-O4 13 62,5 rpm 55 Jamaica 0.28-4 Stephenson et al. (1994) Haake Rotovisco RVI MvMVI 42-571 S1 60 USA 0.1-1 Evans et al. (1986) Rotovisco MVMVII 1-io3 s-'; 59d 23; 60 USA 1-10 Bagley & Christianson (1982) Rotovisco MVMV3 1, 1 s-1 40 Netherlands 10-80 Wanink et al. (1994) Rotovisco RV3 Mv2, SV 22-135 s-' 35 UK 0.1-2 Pavitt (1987) Rotovisco RVl FK/sv2 54, 162rpm 40 Tanzania/Sweden 0-2 Svanberg (1987) Rotovisco RV3 n.r. 256, 362, 512rpm 30, 35, 40 Uganda 0-0.3 mafunda et al. (1997, 1998) VT500 SVDIN 83 s-' 45 France 1-3 Trèche & Mbomé Lapé (1999) Rheomat 30 MS-OMS-DIN45 n.r. n.r. Sweden 0.1-0.4 Karlsson & Svanberg (1982)

' In the measurement conditions used. n.r. = not reported. Table 5. Correspondences between short consistency descriptions and apparent viscosity values measured with the most common viscometers used in previously published studies for two types of gruels (maize flour and multicomponent flour) at different concentrations at 45°C

Empirical measurements

Shear rate well defined Brookjìeld RVT

Haake VT550 Rion VTO4 spindle 6 spindle 4 Gruel spindle I concentration at 8s' at 83 at 800 s-' at 62.5 rpin at 20 rpin at5rpm at20rpm Qualitative description of (d100 6) Pas Pa.s Pas Pa.s Pas at 50 rpin Pas Pa.s gniel consistency

Maize flour 5.91 0.46 0.16 0.04 1.2 1.4 0.6 2.0 drinkable 6.79 0.65 0.21 0.06 2.2 3.1 1.7 12.2 3.4 limit between drinkable and spoonable 8.72 2.63 1.05 0.20 8.5 11.2 5.9 36.1 - spoonable 9.10 3.46 1.37 0.24 10.0 13.9 7.3 - - spoonable-thick 10.3 7.21 2.41 0.39 22.0 40.2 - - - limit between spoonable and too thick to be consumed by infants

Multicomponent flour (millet, soyabean, groundnut, sugar, salt) 9.10 0.37 0.16 0.05 1.1 1.7 0.9 5.3 2.0 drinkable 11.4 1.11 0.54 0.11 2.6 5.4 3.0 14.9 5.5 limit between drinkable and spoonable 12.5 1.71 0.72 0.15 3.4 8.2 4.5 25.8 8.7 spoonable 13.4 2.59 0.89 0.19 6.2 11.1 6.1 35.1 - spoonable-thick 15.7 6.05 1.59 0.37 8.1 26.3 13.1 - - limit between spoonable and too thick to be consrimed by iigants viscosity of gruel for ii$ants 399

rates (10 to 100 s-') in a human's mouth when the Rion VT04 which is sturdy and particularly helshe consumes liquids with a viscosity easy to use. The measurement conditions we ranging from 0.1 to 100 Pa.s (Shama & use are with spindle 1,2 or 3 depending on the Sherman, 1973). Secondly, they reduce the gruel viscosity range, the single rotation speed ." measurement variability associated with the allowed by the apparatus (62.5 rpm) and shear . non-Newtonian nature of gruels: preset shear time of 1min. rate and time (83 s-l and 10min) in order to in large field surveys, where many different take shear-thinning and thixotropic behavior types of gruels can be encountered, it could be into account. suitable to use a rough classification based on a to carry out an experimental study in the field, qualitative description. In this case, it is a simple empirical viscometer in standardized essential to carry out studies in the field to take conditions would be suitable. However, the the mother perception into account for the results obtained will only be of comparative determination of consistency classes and to value for gruels with close compositions. Even use vocabulary that will be fully understood by in the field, measurement conditions (at least, the survey population. It could also be useful spindle number, rotation speed, rotation time to deteimine correspondences between senso- and gruel temperature) have to be standar- rial descriptors and apparent viscosity values dized. In our laboratory, we have chosen to use obtained in standardized conditions.

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